For the sake of better understanding of a prior art pressure control valve to which the present invention pertains, reference is had to FIGS. 17 through 21 of the drawings accompanying herewith.
FIG. 17 exemplifies an air conditioning system which includes an evaporator 3 connected at its inlet to a receiver 1 by way of an expansion valve 2 and at the outlet to the suction side of a refrigerant compressor 5 by way of an evaporator pressure control valve assembly 40, respectively. The discharge side of the compressor 5 is connected to a condenser 6 which is in turn connected to the receiver 1, thus forming a complete refrigeration circuit. The arrows in the diagram of FIG. 17 designate the direction in which the refrigerant flows in the system. If it were not for the pressure control valve assembly 40 in this system, the evaporator 3 itself would be cooled down to be frosted if the pressure at the outlet of the evaporator is kept at a low level. Thus, this control valve assembly 40 performs the function of controlling the refrigerant pressure at the outlet of the evaporator.
The pressure control valve assembly 40 has formed therein a passage A for allowing refrigerant gas evaporated from the evaporator 3 to pass therethrough toward the compressor 5. A movable valve spool or plunger 27 is slidably disposed, cooperating with its associated valve seat portion 8 to provide a variable throttling passage 13 for controlling the pressure of the refrigerant gas from the evaporator 3 in a known way. In the throttling passage 13, the end 27y of plunger 27 adjacent to the seat portion 8 forms substantially a right angle with a cylindrical sealing surface 8x of the seat portion. However, such pressure control valve assembly 40 poses a problem as will be described below.
FIGS. 18 and 19 show positions of the plunger 27 relative to the seat portion 8. FIG. 18 represents a wide-open position of the plunger 27; FIG. 19 shows a position thereof between the wide-open and closed or full-throttling positions. In these two drawings, H.sub.o and H represent the distances of the end surface 27y of the plunger 27 as measured from the seat surface of the seat portion 8, respectively. Therefore, the position of the plunger 27 can be represented by H-to-H.sub.o ratio, or H/H.sub.o.
The outer diameter of the plunger is represented by D.sub.7 and the inner diameter of the cylindrical sealing surface 8x on the seat portion 8 is represented by D.sub.8, respectively. Symbol L (FIG. 18) represents the shortest length or distance as measured between the end surface 27y of the plunger 27 and the seal surface 8x on the seat portion 8. With this information given, the area S.sub.o of the throttling passage 13 with the plunger 27 placed in its wide-open position and the area S of the throttling passage with the plunger moved away from its wide-open position and before the distance L becomes substantially zero can be expressed as L(D.sub.7 +D.sub.8).pi./2, respectively. It is noted that the dimension of the clearance between the sealing surface 8x and the periphery of the plunger end is shown somewhat exaggerated and that the amount of refrigerant gas allowed through this clearance is only to such an extent that lubricating oil mixed with the refrigerant may pass through this clearance for lubrication of the mating surfaces.
Reference is now made to the diagram of FIG. 20 showing the opening of the throttling passage 13 in terms of S-to-S.sub.o ratio (or S/S.sub.o) varying with the movement or variable position of the plunger 27. When the plunger 27 moves from its wide-open position (FIG. 18) where H/H.sub.o is 1.0, to a position where H/H.sub.o is about 0.25, the opening of the throttling passage 13 (S/S.sub.o) is decreased linearly as indicated by line "a" in the diagram of FIG. 20, which means that throttling effect occurs in a linear manner. In the movement of the plunger 27 from the above position (where H/H.sub.o is about 0.25) to its full-throttling position where the end surface 27y of the plunger is positioned very close to the seat surface of the seat portion 8, the throttling effect becomes substantially unchanged a indicated by line "e" of the same diagram.
Then, the variation of the cooling capacity with the varying plunger position (H/H.sub.o) will be described with reference to FIG. 21. As shown in the diagram of the drawing, the cooling capacity, which is indicated by the ratio of the capacity Q for a given plunger position to the full capacity Q.sub.o obtainable when the plunger 27 is in its wide-open position (namely Q/Q.sub.o, is reduced at a very rapid rate during the plunger movement from about 0.5 to about 0.3 of the ratio S/S.sub.o. To describe the diagram of FIG. 21 in other words, the cooling capacity does not show a noticeable decrease until the plunger 27 is moved for a substantial distance to about 0.5 of the ratio S/S.sub.o, but thereafter the capacity is decreased at a very rapid rate in a small range of plunger movement.
Consequently, the pressure P.sub.s of the refrigerant gas evaporating from the evaporator 3 in the passage A is caused to be changed rapidly by a slight movement of the plunger 27, thereby not only making it difficult to achieve stability in the controlling of the evaporating pressure of the refrigerant gas, but also causing chattering of the plunger 27.
Furthermore, in the range of plunger movement between the points about 0.5 and about 0.3, the cooling capacity will have to be controlled by only a slight movement of the plunger 27. This will make it difficult to achieve smooth controlling of the cooling capacity for moderate change in temperature and, therefore, the cooling comfort in the compartment to be cooled will be affected.